Network Working Group M. StJohns
Internet-Draft Nominum, Inc.
Intended status: Informational August 14, 2006
Expires: February 15, 2007
Automated Updates of DNSSEC Trust Anchorsdraft-ietf-dnsext-trustupdate-timers-04
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document describes a means for automated, authenticated and
authorized updating of DNSSEC "trust anchors". The method provides
protection against single key compromise of a key in the trust point
key set. Based on the trust established by the presence of a current
anchor, other anchors may be added at the same place in the
hierarchy, and, ultimately, supplant the existing anchor.
This mechanism will require changes to resolver management behavior
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Internet-Draft trustanchor-update August 20061. Introduction
As part of the reality of fielding DNSSEC (Domain Name System
Security Extensions) [RFC2535] [RFC4033][RFC4034][RFC4035], the
community has come to the realization that there will not be one
signed name space, but rather islands of signed name space each
originating from specific points (i.e. 'trust points') in the DNS
tree. Each of those islands will be identified by the trust point
name, and validated by at least one associated public key. For the
purpose of this document we'll call the association of that name and
a particular key a 'trust anchor'. A particular trust point can have
more than one key designated as a trust anchor.
For a DNSSEC-aware resolver to validate information in a DNSSEC
protected branch of the hierarchy, it must have knowledge of a trust
anchor applicable to that branch. It may also have more than one
trust anchor for any given trust point. Under current rules, a chain
of trust for DNSSEC-protected data that chains its way back to ANY
known trust anchor is considered 'secure'.
Because of the probable balkanization of the DNSSEC tree due to
signing voids at key locations, a resolver may need to know literally
thousands of trust anchors to perform its duties. (e.g. Consider an
unsigned ".COM".) Requiring the owner of the resolver to manually
manage this many relationships is problematic. It's even more
problematic when considering the eventual requirement for key
replacement/update for a given trust anchor. The mechanism described
herein won't help with the initial configuration of the trust anchors
in the resolvers, but should make trust point key replacement/
rollover more viable.
As mentioned above, this document describes a mechanism whereby a
resolver can update the trust anchors for a given trust point, mainly
without human intervention at the resolver. There are some corner
cases discussed (e.g. multiple key compromise) that may require
manual intervention, but they should be few and far between. This
document DOES NOT discuss the general problem of the initial
configuration of trust anchors for the resolver.
1.1. Compliance Nomenclature
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14, [RFC2119].
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Internet-Draft trustanchor-update August 20061.2. Changes since -00
N.B. This section to be deleted prior to submission to RFC editor.
Added the concept of timer triggered resolver queries to refresh the
resolvers view of the trust anchor key RRSet.
Re-submitted expired draft as -01. Updated DNSSEC RFC References.
Draft -02. Added the IANA Considerations section. Added text to
describe what happens if all trust anchors at a trust point are
deleted.
Draft -03. Revised the trust point deletion language to note
limitations.
Draft -04. Restructured section 4.3 (Trust point deletion) and 5
(Scenarios). Section 4.3 is now section 5. Section 5 is now section6 and "Informative"
2. Theory of Operation
The general concept of this mechanism is that existing trust anchors
can be used to authenticate new trust anchors at the same point in
the DNS hierarchy. When a new SEP key (see [RFC4034] section 2.1.1)
is added to a trust point DNSKEY RRSet, and when that RRSet is
validated by an existing trust anchor, then the new key can be added
to the set of trust anchors.
There are some issues with this approach which need to be mitigated.
For example, a compromise of one of the existing keys could allow an
attacker to add their own 'valid' data. This implies a need for a
method to revoke an existing key regardless of whether or not that
key is compromised. As another example assuming a single key
compromise, an attacker could add a new key and revoke all the other
old keys.
2.1. Revocation
Assume two trust anchor keys A and B. Assume that B has been
compromised. Without a specific revocation bit, B could invalidate A
simply by sending out a signed trust point key set which didn't
contain A. To fix this, we add a mechanism which requires knowledge
of the private key of a DNSKEY to revoke that DNSKEY.
A key is considered revoked when the resolver sees the key in a self-
signed RRSet and the key has the REVOKE bit (see Section 7 below) set
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to '1'. Once the resolver sees the REVOKE bit, it MUST NOT use this
key as a trust anchor or for any other purposes except validating the
RRSIG over the DNSKEY RRSet specifically for the purpose of
validating the revocation. Unlike the 'Add' operation below,
revocation is immediate and permanent upon receipt of a valid
revocation at the resolver.
A self-signed RRSet is a DNSKEY RRSet which contains the specific
DNSKey and for which there is a corresponding validated RRSIG record.
It's not a special DNSKEY RRSet, just a way of describing the
validation requirements for that RRSet.
N.B. A DNSKEY with the REVOKE bit set has a different fingerprint
than one without the bit set. This affects the matching of a DNSKEY
to DS records in the parent, or the fingerprint stored at a resolver
used to configure a trust point.
In the given example, the attacker could revoke B because it has
knowledge of B's private key, but could not revoke A.
2.2. Add Hold-Down
Assume two trust point keys A and B. Assume that B has been
compromised. An attacker could generate and add a new trust anchor
key - C (by adding C to the DNSKEY RRSet and signing it with B), and
then invalidate the compromised key. This would result in the both
the attacker and owner being able to sign data in the zone and have
it accepted as valid by resolvers.
To mitigate, but not completely solve, this problem, we add a hold-
down time to the addition of the trust anchor. When the resolver
sees a new SEP key in a validated trust point DNSKEY RRSet, the
resolver starts an acceptance timer, and remembers all the keys that
validated the RRSet. If the resolver ever sees the DNSKEY RRSet
without the new key but validly signed, it stops the acceptance
process and resets the acceptance timer. If all of the keys which
were originally used to validate this key are revoked prior to the
timer expiring, the resolver stops the acceptance process and resets
the timer.
Once the timer expires, the new key will be added as a trust anchor
the next time the validated RRSet with the new key is seen at the
resolver. The resolver MUST NOT treat the new key as a trust anchor
until the hold down time expires AND it has retrieved and validated a
DNSKEY RRSet after the hold down time which contains the new key.
N.B.: Once the resolver has accepted a key as a trust anchor, the key
MUST be considered a valid trust anchor by that resolver until
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explictly revoked as described above.
In the given example, the zone owner can recover from a compromise by
revoking B and adding a new key D and signing the DNSKEY RRSet with
both A and B.
The reason this does not completely solve the problem has to do with
the distributed nature of DNS. The resolver only knows what it sees.
A determined attacker who holds one compromised key could keep a
single resolver from realizing that key had been compromised by
intercepting 'real' data from the originating zone and substituting
their own (e.g. using the example, signed only by B). This is no
worse than the current situation assuming a compromised key.
2.3. Remove Hold-down
A new key which has been seen by the resolver, but hasn't reached
it's add hold-down time, MAY be removed from the DNSKEY RRSet by the
zone owner. If the resolver sees a validated DNSKEY RRSet without
this key, it waits for the remove hold-down time and then, if the key
hasn't reappeared, SHOULD discard any information about the key.
2.4. Active Refresh
A resolver which has been configured for automatic update of keys
from a particular trust point MUST query that trust point (e.g. do a
lookup for the DNSKEY RRSet and related RRSIG records) no less often
than the lesser of 15 days or half the original TTL for the DNSKEY
RRSet or half the RRSIG expiration interval. The expiration interval
is the amount of time from when the RRSIG was last retrieved until
the expiration time in the RRSIG.
If the query fails, the resolver MUST repeat the query until
satisfied no more often than once an hour and no less often than the
lesser of 1 day or 10% of the original TTL or 10% of the original
expiration interval.
2.5. Resolver Parameters2.5.1. Add Hold-Down Time
The add hold-down time is 30 days or the expiration time of the TTL
of the first trust point DNSKEY RRSet which contained the key,
whichever is greater. This ensures that at least two validated
DNSKEY RRSets which contain the new key MUST be seen by the resolver
prior to the key's acceptance.
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Internet-Draft trustanchor-update August 20062.5.2. Remove Hold-Down Time
The remove hold-down time is 30 days.
2.5.3. Minimum Trust Anchors per Trust Point
A compliant resolver MUST be able to manage at least five SEP keys
per trust point.
3. Changes to DNSKEY RDATA Wire Format
Bit n [msj2] of the DNSKEY Flags field is designated as the 'REVOKE'
flag. If this bit is set to '1', AND the resolver sees an
RRSIG(DNSKEY) signed by the associated key, then the resolver MUST
consider this key permanently invalid for all purposes except for
validing the revocation.
4. State Table
The most important thing to understand is the resolver's view of any
key at a trust point. The following state table describes that view
at various points in the key's lifetime. The table is a normative
part of this specification. The initial state of the key is 'Start'.
The resolver's view of the state of the key changes as various events
occur.
This is the state of a trust point key as seen from the resolver.
The column on the left indicates the current state. The header at
the top shows the next state. The intersection of the two shows the
event that will cause the state to transition from the current state
to the next.
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NEXT STATE
--------------------------------------------------
FROM |Start |AddPend |Valid |Missing|Revoked|Removed|
----------------------------------------------------------
Start | |NewKey | | | | |
----------------------------------------------------------
AddPend |KeyRem | |AddTime| | |
----------------------------------------------------------
Valid | | | |KeyRem |Revbit | |
----------------------------------------------------------
Missing | | |KeyPres| |Revbit | |
----------------------------------------------------------
Revoked | | | | | |RemTime|
----------------------------------------------------------
Removed | | | | | | |
----------------------------------------------------------
State Table
4.1. Events NewKey The resolver sees a valid DNSKEY RRSet with a new SEP key.
That key will become a new trust anchor for the named trust point
after its been present in the RRSet for at least 'add time'.
KeyPres The key has returned to the valid DNSKEY RRSet.
KeyRem The resolver sees a valid DNSKEY RRSet that does not contain
this key.
AddTime The key has been in every valid DNSKEY RRSet seen for at
least the 'add time'.
RemTime A revoked key has been missing from the trust point DNSKEY
RRSet for sufficient time to be removed from the trust set.
RevBit The key has appeared in the trust anchor DNSKEY RRSet with
its "REVOKED" bit set, and there is an RRSig over the DNSKEY RRSet
signed by this key.
4.2. States Start The key doesn't yet exist as a trust anchor at the resolver.
It may or may not exist at the zone server, but hasn't yet been
seen at the resolver.
AddPend The key has been seen at the resolver, has its 'SEP' bit
set, and has been included in a validated DNSKEY RRSet. There is
a hold-down time for the key before it can be used as a trust
anchor.
Valid The key has been seen at the resolver and has been included in
all validated DNSKEY RRSets from the time it was first seen up
through the hold-down time. It is now valid for verifying RRSets
that arrive after the hold down time. Clarification: The DNSKEY
RRSet does not need to be continuously present at the resolver
(e.g. its TTL might expire). If the RRSet is seen, and is
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validated (i.e. verifies against an existing trust anchor), this
key MUST be in the RRSet otherwise a 'KeyRem' event is triggered.
Missing This is an abnormal state. The key remains as a valid trust
point key, but was not seen at the resolver in the last validated
DNSKEY RRSet. This is an abnormal state because the zone operator
should be using the REVOKE bit prior to removal. [Discussion
item: Should a missing key be considered revoked after some period
of time?]
Revoked This is the state a key moves to once the resolver sees an
RRSIG(DNSKEY) signed by this key where that DNSKEY RRSet contains
this key with its REVOKE bit set to '1'. Once in this state, this
key MUST permanently be considered invalid as a trust anchor.
Removed After a fairly long hold-down time, information about this
key may be purged from the resolver. A key in the removed state
MUST NOT be considered a valid trust anchor.
5. Trust Point Deletion
A trust point which has all of its trust anchors revoked is
considered deleted and is treated as if the trust point was never
configured. If there are no superior configured trust points, data
at and below the deleted trust point are considered insecure by the
resolver. If there ARE superior configured trust points, data at and
below the deleted trust point are evaluated with respect to the
superior trust point.
Alternately, a trust point which is subordinate to another configured
trust point MAY be deleted by a resolver after 180 days where such
trust point validly chains to a superior trust point. The decision
to delete the subordinate trust anchor is a local configuration
decision. Once the subordinate trust point is deleted, validation of
the subordinate zone is dependent on validating the chain of trust to
the superior trust point.
6. Scenarios - Informative
The suggested model for operation is to have one active key and one
stand-by key at each trust point. The active key will be used to
sign the DNSKEY RRSet. The stand-by key will not normally sign this
RRSet, but the resolver will accept it as a trust anchor if/when it
sees the signature on the trust point DNSKEY RRSet.
Since the stand-by key is not in active signing use, the associated
private key may (and should) be provided with additional protections
not normally available to a key that must be used frequently. E.g.
locked in a safe, split among many parties, etc. Notionally, the
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stand-by key should be less subject to compromise than an active key,
but that will be dependent on operational concerns not addressed
here.
6.1. Adding A Trust Anchor
Assume an existing trust anchor key 'A'.
1. Generate a new key pair.
2. Create a DNSKEY record from the key pair and set the SEP and Zone
Key bits.
3. Add the DNSKEY to the RRSet.
4. Sign the DNSKEY RRSet ONLY with the existing trust anchor key -
'A'.
5. Wait a while.
6. The new trust anchor will be populated at the resolvers on the
schedule described by the state table and update algorithm - see
Section 2 above
6.2. Deleting a Trust Anchor
Assume existing trust anchors 'A' and 'B' and that you want to revoke
and delete 'A'.
1. Set the revolcation bit on key 'A'.
2. Sign the DNSKEY RRSet with both 'A' and 'B'.
'A' is now revoked. The operator SHOULD include the revoked 'A' in
the RRSet for at least the remove hold-down time, but then may remove
it from the DNSKEY RRSet.
6.3. Key Roll-Over
Assume existing keys A and B. 'A' is actively in use (i.e. has been
signing the DNSKEY RRSet.) 'B' was the stand-by key. (i.e. has been
in the DNSKEY RRSet and is a valid trust anchor, but wasn't being
used to sign the RRSet.)
1. Generate a new key pair 'C'.
2. Add 'C' to the DNSKEY RRSet.
3. Set the revocation bit on key 'A'.
4. Sign the RRSet with 'A' and 'B'.
'A' is now revoked, 'B' is now the active key, and 'C' will be the
stand-by key once the hold-down expires. The operator SHOULD include
the revoked 'A' in the RRSet for at least the remove hold-down time,
but may then remove it from the DNSKEY RRSet.
6.4. Active Key Compromised
This is the same as the mechanism for Key Roll-Over (Section 6.3)
above assuming 'A' is the active key.
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Internet-Draft trustanchor-update August 20066.5. Stand-by Key Compromised
Using the same assumptions and naming conventions as Key Roll-Over
(Section 6.3) above:
1. Generate a new key pair 'C'.
2. Add 'C' to the DNSKEY RRSet.
3. Set the revocation bit on key 'B'.
4. Sign the RRSet with 'A' and 'B'.
'B' is now revoked, 'A' remains the active key, and 'C' will be the
stand-by key once the hold-down expires. 'B' SHOULD continue to be
included in the RRSet for the remove hold-down time.
6.6. Trust Point Deletion
To delete a trust point which is subordinate to another configured
trust point (e.g. example.com to .com) requires some juggling of the
data. The specific process is:
1. Generate a new DNSKEY and DS record and provide the DS record to
the parent along with DS records for the old keys
2. Once the parent has published the DSs, add the new DNSKEY to the
RRSet and revoke ALL of the old keys at the same time while
signing the DNSKEY RRSet with all of the old and new keys.
3. After 30 days stop publishing the old, revoked keys and remove
any corresponding DS records in the parent.
Revoking the old trust point keys at the same time as adding new keys
that chain to a superior trust prevents the resolver from adding the
new keys as trust anchors. Adding DS records for the old keys avoids
a race condition where either the subordinate zone becomes unsecure
(because the trust point was deleted) or becomes bogus (because it
didn't chain to the superior zone).
7. IANA Considerations
The IANA will need to assign a bit in the DNSKEY flags field (see
section 4.3 of [RFC3755]) for the REVOKE bit. There are no other
IANA actions required.
8. Security Considerations8.1. Key Ownership vs Acceptance Policy
The reader should note that, while the zone owner is responsible
creating and distributing keys, it's wholly the decision of the
resolver owner as to whether to accept such keys for the
authentication of the zone information. This implies the decision
update trust anchor keys based on trust for a current trust anchor
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key is also the resolver owner's decision.
The resolver owner (and resolver implementers) MAY choose to permit
or prevent key status updates based on this mechanism for specific
trust points. If they choose to prevent the automated updates, they
will need to establish a mechanism for manual or other out-of-band
updates outside the scope of this document.
8.2. Multiple Key Compromise
This scheme permits recovery as long as at least one valid trust
anchor key remains uncompromised. E.g. if there are three keys, you
can recover if two of them are compromised. The zone owner should
determine their own level of comfort with respect to the number of
active valid trust anchors in a zone and should be prepared to
implement recovery procedures once they detect a compromise. A
manual or other out-of-band update of all resolvers will be required
if all trust anchor keys at a trust point are compromised.
8.3. Dynamic Updates
Allowing a resolver to update its trust anchor set based in-band key
information is potentially less secure than a manual process.
However, given the nature of the DNS, the number of resolvers that
would require update if a trust anchor key were compromised, and the
lack of a standard management framework for DNS, this approach is no
worse than the existing situation.
9. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2535] Eastlake, D., "Domain Name System Security Extensions",
RFC 2535, March 1999.
[RFC3755] Weiler, S., "Legacy Resolver Compatibility for Delegation
Signer (DS)", RFC 3755, May 2004.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, March 2005.
[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security Extensions",
RFC 4034, March 2005.
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